It is well known that speckle limits the contrast and resolution of images obtained when a laser is used as a light source in a high-resolution imaging system, such as in clinical diagnostic microscopes and confocal microscopes.
Most attempts to reduce the impact of laser speckle on microscopy have involved continuous-wave (CW) laser sources, and a movable optical component, such as a rotating diffuser. This approach does not eliminate speckle but instead reduces the intensity variations characteristic of speckle in an image by averaging the variations over long timescales (typically milliseconds or more). One disadvantage to the rotating diffuser (and other movable components) is that moving parts are required in the imaging system. Consequently, the integration times required to obtain an acceptable image must be sufficiently long to reduce intensity variations to a desired level. Therefore, this approach is incapable of imaging objects on a time scale that is less than the required averaging time. This is a serious liability if one wishes to image cells and other organisms that move or change shape with time. Another clear difficulty of incorporating movable components into a system is that such mechanical mechanisms adversely impact cost and the reliability of the imaging system.
A vibrating fiber represents one conventional approach that requires moving parts and a similar integration time. See, R. Voelkel, K. J. Weible, “Laser beam homogenizing: limitations and constraints” Proc. SPIE 7102, 71020J-1 (2008). This technique also creates difficulties for the design of the light source and its associated optics because coupling light into a fiber requires precision alignment.
Other approaches to reducing speckle include chaotic cavities and random lasers. See, respectively, B. Redding et al. “Low spatial coherence electrically pumped semiconductor laser for speckle-free full-field imaging,” PNAS 112, 1304 (2015); B. Redding et al., “Speckle-free laser imaging using random laser illumination,” Nature Photonics 6, 355 (2012). These two types of sources often suffer from low light-collection efficiencies and high divergence of the emitted optical radiation because of the random nature of the lasers. Furthermore, random lasers are known to produce different emission spectra from shot-to-shot.
Still other approaches include passing the beam through nonlinear media and multi-mode optical fibers with a complex optical arrangement to reduce the spatial and temporal coherence of the light. See, D. Kohler, et al. Speckle reduction in pulsed-laser photographs. Opt. Commun. 12, 24 (1974); J. P. Huignard et al., “Speckle-free imaging in four-wave mixing experiments with Bi12SiO20 crystals. Opt. Lett. 5, 436 (1980). While somewhat effective when narrowband light is not required, the complex optical designs are costly and present other alignment problems. These sources are also difficult to integrate into the compact package required by various high-resolution imaging systems and microscope-based imaging systems, in particular.
Arrays of continuous wave, vertical cavity surface-emitting lasers (VCSELs) have also been utilized to reduce speckle. However, VCSELs suffer from high diffraction losses, as well as limited bandwidth (owing to the cavity size), a limited selection of array geometries, and a limited selection of available gain media.
Because of these drawbacks to the use of conventional lasers, state-of-the-art clinical microscopy systems often rely upon incoherent light sources such as LEDs. Standard lamps and LED light sources provide comparatively low intensity but are frequently employed in place of lasers to avoid the difficulties associated with speckle. In addition to having severely restricted intensities, conventional lamps and LEDs generally provide CW (rather than pulsed) light and are, therefore, unsuitable for capturing images on a short timescale.
U.S. Pat. No. 7,339,148 to Kawano et al. discloses a confocal microscope that can use a laser light source, and lamps such as xenon or halogen lamps. The laser light or other illumination light is modulated by a digital mirror array. The laser light source can include a plurality of lasers having differing wavelengths, but this microscope requires spatial scanning of the illumination light with a galvanometer-driven mirror. In operation, the micromirrors scan a laser spot over the sample. This allows the computer to construct the image point-by-point as the micromirrors scan the beam. When the microscope is operated in reflectance mode (for imaging “label-free” samples that have no dye in them to generate fluorescence), there will be speckle.
U.S. Pat. No. 7,030,383 to Babayoff et al. describes a confocal imaging apparatus incorporating a laser, and capable of imaging specimens that are not flat. This apparatus also requires an optical component in the system (an imaging optic) to be movable in order for the speckle produced by the laser to be reduced.